1. Field of the Invention
The present invention relates to a transmitting apparatus used in a wireless transmission system, and particularly to a transmitting apparatus suitable for use in a transmission system which places importance on a reduction in power consumption as an infra-facility.
2. Description of the Related Art
In an electronic device that operates in an analog region, the existence of a nonlinear characteristic is almost inevitable, and its input/output characteristic contains a linear region and a nonlinear region. This particularly presents a problem for a main amplifier (corresponding to a final-stage power amplifier for an RF signal) of a wireless communication transmitting apparatus.
In the case of such an apparatus, the output of the main amplifier is supplied to an antenna and transmitted by radio. Therefore, when a nonlinear region exists in the output, a harmonic due to nonlinear distortion is produced and exerts an influence on a transmission system in an adjacent frequency band as spurious radiation, thereby causing a fear of interfering with other transmission system. Therefore, the nonlinear distortion is placed under strict control.
Meanwhile, in order to suppress the nonlinear distortion, the electronic device such as the main amplifier may be used in a linear region alone. Since, however, the linear region generally stays within a range in which an input level is low, power efficiency (output power/power consumption) of the amplifier is reduced where it is operated only in the linear region. Consequently, energy saving cannot be obtained in the case of the main amplifier.
Thus, a large challenge for a transmitting apparatus is how to improve power efficiency while suppressing the nonlinear distortion. On the other hand, as a technique that will bring about the attainment of this challenge, there is first considered the application of a limiter (refer to, for example, a patent document 1 (Japanese Patent Laid-Open No. 2003-46480) or (Japanese Patent Laid Open No. 2003-258683). Now, the present limiter means an amplitude limiting circuit which is also called limiter. In the present specification, however, it will be explained as one unified as the limiter.
FIG. 3(a) shows one example of a transmitting apparatus according to a prior art, which is equipped with a limiter. As illustrated in the figure, the transmitting apparatus comprises a modulator 101, a limiter 102 which suppresses peak power of a modulated signal, a frequency converter 103 which up-converts a modulated signal (or digital IF signal) of a baseband to a radio frequency band (converts it to a high frequency), and an amplifier 104 which amplifies a transmit signal lying in a radio frequency band to a desired signal level and supplies it to an antenna. Thus, the present amplifier 104 corresponds to the main amplifier referred to above.
In this case, the limiter 102 functions so as to limit the amplitude of the modulated signal outputted from the modulator 101 to a certain predetermined voltage (called threshold value Vth) and suppress a momentary high level region (peak level) that appears in the input of the amplifier 104 as shown in FIG. 4.
The effect shown in FIG. 4, of the limiter, which can be confirmed in another form, corresponds to an appearance probability characteristic shown in FIG. 5. In the present figure, the horizontal axis indicates a peak to average power ratio (PAPR: abbreviation of Peak to Average Power Ratio), and the vertical axis indicates appearance probability. It is well understood from the characteristic of FIG. 5 that the peak power has been suppressed by the limiter as is apparent from comparison between characteristics prior and subsequent to the passage thereof through the limiter.
The relationship between the nonlinearity of such an amplifier and the limiter is further well-shown in FIG. 6. FIG. 6(a) shows a case in which no limiter is provided, and FIG. 6(b) shows a case in which the limiter exists. Referring first to FIG. 6(a), a nonlinear region of an input/output characteristic of the amplifier is made active when signal input power reaches a peak, so that nonlinear distortion occurs.
On the other hand, when the limiter exists as shown in FIG. 6(b), the peak of the signal input power is suppressed by the limiter. Therefore, only a linear region of an input/output characteristic of the main amplifier can be made active. As a result, since the nonlinear distortion is reduced, the operating point of the input/output characteristic can be deepened correspondingly, thus making it possible to improve the power efficiency of the amplifier.
Next, as a technique that contributes to an improvement in power efficiency of the amplifier, may be mentioned the application of a dynamic bias technique in addition to the above (refer to, for example, a patent document 3 (Japanese Patent Laid-Open No. 2002-176368) or 4 (Japanese Patent Publication No. 2004-500781)).
Now, FIG. 3(b) shows one example of a transmitting apparatus according to a prior art, to which the present dynamic bias technique is applied. As shown in the figure, the present transmitting apparatus is identical to the prior art of FIG. 3(a) in that the former is provided with a modulator 101, a frequency converter 103 that up-converts a modulated signal (or digital IF signal) of a baseband, and an amplifier 104 which amplifies a transit signal lying in a radio frequency band to a desired signal level.
In FIG. 3(b), however, a controller 105 is further provided. On the other hand, the limiter 102 shown in FIG. 3(a) is omitted.
As shown in the figure, the controller 105 is provided with a level determinater 105A and a bias setter 105B. The level determinater 105A determines (detects) an average level of a modulated signal outputted from the modulator 101. Then, the bias setter 105B sets bias of the amplifier 104 in accordance with the result of determination (detection) thereby to obtain control by the dynamic bias technique, i.e., dynamic bias control.
Here, the dynamic bias control, if described intelligibly, means control for operating an amplifier whose original maximum output is, for example, 100 W, as an amplifier whose maximum output is 50 W, or operating it as an amplifier whose maximum output is 10 W.
Let's now assume that there is an amplifier having such an input/output characteristic as shown in FIG. 7(a), and when the average level of an input signal is increased to a saturated region of the input/output characteristic in this case, i.e., the amplifier is excited up to the full dynamic range, the maximum output, i.e., the original maximum output is 100 W, for example.
Assuming, in this case, that as shown in the figure, the average level of the input signal is low and an operating range does not reach a saturation point, the expected dynamic range is not utilized and goes to not only waste but also a loss in power consumption. Thus, this runs counter to an improvement in power efficiency.
Thus, in such a case, a source voltage or power supply voltage (or source current) of the amplifier is restricted according to the average level of the modulated signal outputted from the modulator 101 to lower the saturation point of the input/output characteristic as shown in FIG. 7(b) and set the maximum output to, for example, 50 W. If done in this way, then power consumption is suppressed and the power efficiency can be improved.
This is called dynamic bias control. In brief, it indicates such control that the source voltage (or source current) of the amplifier is controlled according to the input level and the maximum output of the amplifier is restricted depending upon the average level of the input.
The dynamic bias control is generally utilized in mobile communications or the like where the average level of the input signal greatly changes. This is because the traffic is low from midnight to early morning or the like and the average level of a signal at this time is commonly far inferior to the pre-estimated saturation power (corresponding to the maximum output impossible to output any longer) of the amplifier.
It will be also described intelligibly. Since the traffic increases in the daytime, for example, the average level of the input is also high and hence an output of 50 W, for example, is required. When, however, the average level is reduced late at night and even an output of 1 W is enough, it is useless to use the 100W amplifier as it is.
Thus, in this case, the source voltage or current of the amplifier is restricted by the dynamic bias control and the amplifier is operated as the 100 W amplifier when 50 W is required, for example. If the amplifier is controlled in such a manner that it is operated as a 10 W amplifier when 1 W is required, then power consumption is reduced, thus making it possible to improve power efficiency and make a contribution to energy saving.
Meanwhile, since there is no limitation to energy saving, a transmitting apparatus utilizing the above limiter and dynamic bias control in combination has been estimated as the prior art as shown in FIG. 3(c) with a view of aiming at a further reduction in power consumption. This is however equivalent to one wherein the transmitting apparatus shown in FIG. 3(b) is provided with the limiter 102 of the transmitting apparatus shown in FIG. 3(a). Other configurations remain unchanged.
However, the transmitting apparatus of FIG. 3(c) utilizing the limiter and dynamic bias control in combination in this way involves problems to be described below.
As already described, when power (or voltage) to be handled exceeds the threshold value, the limiter restricts the exceeded power (or voltage). Therefore, the limiter essentially corresponds to the nonlinear device itself. When the power (or voltage) level is increased where the power (or voltage) is greatly restricted, the rate of distortion of a signal increases and hence the prescribed signal quality cannot be maintained.
That is, since the distortion rate increases when the threshold value of the limiter 102 is reduced in the prior art, there is a limit to a reduction in threshold value although it is done to increase the efficiency of the amplifier. Thus, the threshold value should unavoidably be set to a threshold value corresponding to a value larger than some degree in the case of the transmitting apparatus shown in FIG. 3(c).
Therefore, a problem arises in that when the dynamic bias control and the limiter are thus utilized in combination in a state in which the threshold value of the limiter is above some degree, in other words, the limitation made by the limiter is not so effective, the amplitude of the signal is inputted up to the saturation point of the input/output characteristic reduced by the dynamic bias control as shown in FIG. 8, thereby generating large nonlinear distortion.